The goal of our work is to understand sound transmission in normal, diseased and reconstructed middle ears so as to develop better diagnostic tests and surgical procedures for patients with middle-ear disease. Middle ear diseases, such as chronic otitis media and otosclerosis, which affect over 30 million people in the U.S., are common causes of significant conductive hearing loss that range in severity up to 60 dB. Hearing losses of 30-60 dB have significant adverse effects on patients' lives and their ability to communicate. Many aspects of middle ear sound transmission are not well understood. Additionally, hearing results after certain types of middle-ear surgical procedures (especially for chronic otitis media) are often unsatisfactory, because the structural factors that are important for good hearing results are not all that clear. Over the past 10 years, we have utilized a unique and powerful combination of methods to study middle-ear mechanics including in-vivo measurements using laser Doppler vibrometry, in-vitro measurements in cadaveric human temporal bones, and physics-based, quantitative modeling. Our work has a) provided insight into mechanisms of conductive hearing loss caused by a variety of pathologies affecting the middle- and inner- ears, b) resulted in new diagnostic concepts, and c) provided specific surgical recommendations to optimize postoperative hearing results in certain types of middle-ear surgical procedures. Over the next five years, we aim to exploit these methods and use new tools such as external-ear acoustic reflectance and laser holography of motion of the tympanic membrane in order to: a) investigate correlations between ear canal reflectance and umbo velocity in normal and pathologic ears, b) define critical structural features that determine postoperative hearing results in aerated ears after ossicular reconstructions, and c) investigate use of a novel implant to improve post-surgical hearing results in non-aerated ears. We anticipate that our work will lead to better understanding of structure-function relationships in normal and pathological middle ears, improved differential diagnosis of conductive hearing loss, and optimization of surgical techniques and hearing results.